The liquid or solid wastes and by-products or natural or wastewater(s) or water(s) present as described above may be derived from a multitude of sources. Examples of these sources may include:                Oxidation of sulphide-containing soils to form acid sulphate soils (ASS) and acidic water(s) by natural processes (e.g. seasonal changes in groundwater level and/or oxygen status) or soil or rock disturbance (e.g. during construction or excavation)        Industrial processes (e.g. pyrite oxidation, sulphuric acid production) with offsite loss via soil/groundwater infiltration or via natural or artificial drainages of water(s)        Discharge, escape and infiltration of acidic, neutral or alkaline surface water(s) from mining or extractive metallurgical operations        In-situ leaching of orebodies (e.g. uranium or copper ores)        Liquid or solid wastes and by-products and natural and wastewaters such as surface, groundwater and porewater, wastewaters derived from mineral processing (e.g. alkaline red mud via the Bayer process, mineral processing of uranium or copper ores) or water(s) or tailings contained within or derived from tailings storage facilities, storages, containers or other impoundments.        Injected, formation or aquifer waters, derived or contaminated waters, or combinations thereof, derived or obtained from the extraction of one or more of oil, gas, coal seam gas extraction and recovery or associated petrochemical operations including refining, distillation, gas- or oil-water phase separation and water purification and contaminant removal and reuse including injection or re-injection into aquifers, oil or gas deposits, evaporation, irrigation or environmental reuse or discharge.        It is also well recognized that liquid or solid wastes and wastewaters derived from the operation and maintenance of nuclear power plants, nuclear weapons manufacture or decommissioning, nuclear fuel enrichment or processing, nuclear research facilities or similar facilities often represent a major challenge in terms of conversion to stable solid phase materials for transport and storage or reprocessing of constituent radionuclides and other elements represent a seemingly intractable problem particularly if viewed in the context of secure and physically and chemically robust short-term to long-term geological repository.        
As a consequence of the processes that lead to the formation of these liquid and solid waste and by-products and natural and wastewater(s) they may often be enriched in a variety of metals, metalloids and anions, the concentrations of which may exceed both ANZECC Soil and Water Quality guidelines (ANZECC/NHMRC, 1992). In addition, liquid and solid waste and by-products and natural and wastewater(s) containing radionuclides particularly from the uranium mining and processing (e.g. U, Th, Ra, etc) or nuclear power, weapons and/or research industries (U, Ra, Pu, Tc etc) may also be produced with a requirement for safe short- to long-term storage, preferably using LDH or HT chemistry.
Thus, a challenge exists to identify methods for remediation of liquid and solid waste and by-products and natural and wastewater(s) that are both cost-effective and environmentally robust with safe and efficient immobilization (and if appropriate, off-site disposal) of the contaminants after neutralization. Effective long-term management of liquid and solid waste and by-products and natural and wastewater(s) containing a range of contaminants including metals, metalloids and organics and radionuclides is also required to meet regulatory requirements.
Layered double hydroxides (LDH) are a class of both naturally occurring and synthetically produced materials characterised by a positively-charged mixed metal hydroxide layers separated by interlayers that contain water molecules and a variety of exchangeable anions. A LDH is most commonly formed by the co-precipitation of divalent (e.g. Mg2+, Fe2+) and trivalent (e.g. Al3+, Fe3±) metal cation solutions at moderate to high pH (Taylor, 1984, Vucelic et al, 1997, Shin et al, 1996).
The LDH or HT may be used for the removal of a wide range of inorganic and organic contaminants including radionuclides from liquid and solid waste and by-products and natural and wastewater(s). In addition, the LDH or HT may be utilised as a repository of elements or components of solid wastes including contaminants including radionuclides that have been dissolved (e.g. by acid or alkali) and precipitated or recrystallised using the method/chemistry described here.
A LDH compound may be represented by the general formula (1):M(1-X)2+Mx3+(OH)2An−yH2O  (1)
where M2+ and M3+ are divalent and trivalent metal ions, respectively and An− is the interlayer ion of valence n. The x value represents the proportion of trivalent metal ion to the proportion of total amount metal ion and y denotes variable amounts of interlayer water.
Common forms of LDH comprise Mg2+ and Al3+ (commonly known as HT) and Mg2+ and Fe3+ (known as pyroaurites), but other cations, including Ni, Zn, Mn, Ca, Cr and La, are known. The amount of surface positive charge generated is dependant upon the mole ratio of the metal ions in the lattice structure and the conditions of preparation as they affect crystal formation.
The formation of HT (the most commonly synthesised LDH frequently with carbonate as the principal “exchangeable” anion) may be most simply described by the following reaction:6MgCl2+2AlCl3+16NaOH+H2CO3→Mg6Al2(OH)16CO3.nH2O+2HCl
Typically, ratios of divalent to trivalent cations in Hydrotalcites vary from 2:1 to 3:1. Other synthetic pathways to form HT (and other LDH) include synthesis from Mg(OH)2 (brucite) and MgO (calcined magnesia) via neutralisation of acidic solutions (eg. Albiston et al, 1996). This can be described by the following reaction:6Mg(OH)2+2Al(OH)3+2H2SO4→Mg6Al2(OH)16SO4.nH2O+2H2O
A range of metals of widely varying concentrations may also be simultaneously co-precipitated, hence forming a polymetallic LDH. HT or LDH were first described over 60 years ago (Frondel, 1941, Feitknecht, 1942). Sometimes, they can also occur in nature as accessory minerals in soils and sediments (eg. Taylor and McKenzie, 1980). Layered double hydroxides may also be synthesised from industrial waste materials by the reaction of bauxite residue derived from alumina extraction (red mud) with seawater (eg. Thornber and Hughes, 1987), as described by the following reaction:6Mg(OH)2+2Al(OH)3+2Na2CO3→Mg6Al2(OH)16CO3.nH2O+2NaOHor by the reaction of lime with fly ash derived from fossil fuel (eg. coal fired power stations, Reardon and Della Valle, 1997).
Within the LDH or HT structure there are octahedral metal hydroxide sheets that carry a net positive charge due to limited substitution of trivalent for divalent cations as described above. As a consequence, it is possible to substitute a wide range of inorganic or organic anions into the LDH or HT structure. These anions are often referred to as “interlayer anions” as they fit between the layers of hydroxide material. Layered double hydroxides are generally unstable below a pH of approximately 5 (Ookubu et al, 1993) but may act as buffers over a wide range of solution pH (Seida and Nakano, 2002). Layered double hydroxides or HT, and in particular those that contain carbonate as the predominant anion, have also been demonstrated to have a considerable capacity to neutralise a range of mineral acids via consumption of both the hydroxyl and carbonate anions contained within the LDH structure (eg. Kameda et al, 2003).
A number of studies have been conducted to investigate ways to exploit the anion exchange properties of LDH. These studies have focussed on the removal of phosphate and other oxyanions and humic substances from natural and wastewater(s) (Miyata, 1980, Misra and Perrotta, 1992, Amin and Jayson, 1996, Shin et al, 1996, Seida and Nakano, 2000). Phosphate is one of the many anions that may be exchanged into the interlayer space in LDH. Laboratory studies of phosphate uptake using synthetically prepared Mg—Al HT and a range of initial dissolved phosphate concentrations indicate an uptake capacity of from ca. 25-30 mg P/g (Miyata, 1983, Shin et al, 1996) to ca. 60 mg P/g with uptake also influenced by initial phosphate concentration, pH (with maximum phosphate absorption near pH 7), degree of crystallinity and the HT chemistry (Ookubo et al, 1993). A major obstacle to the use of HT for phosphate removal in natural and/or wastewaters is the selectivity for carbonate over phosphate, with a selectivity series in the approximate order CO32−>HPO42−>>SO42−, OH−>F−>Cl−>NO3− (Miyata, 1980, 1983, Sato et al, 1986, Shin et al, 1986, Cavani et al, 1991). Many HT are also synthesised with carbonate as the predominant anion and thus require anion exchange before they are exposed to phosphate. When carbonate is also combined with sulphate, nitrate and chloride (as might commonly occur in natural or wastewaters) the reduction of phosphate absorption to the HT is further decreased (Shin et al, 1996).
A number of recent studies have focussed on the formation and study of synthetic LDH or specifically HT or similar and their subsequent reactivity to a range of anions, particularly silicate (e.g. Depege et al, 1996) with a view to forming polymetallic aluminosilicates, which as potential precursors to clay materials, are thought to limit metal mobility and bioavailability (eg. Ford et al, 1999). A potential also exists for the co-precipitation of silicate and aluminate anions as another precursor of analogue of clay minerals.
Thus, other structural elements or interlayer ions may be incorporated (both inorganic and organic) to assist in both substitution and/or incorporation of ions from solution and/or increased stability. Subsequent formation of chlorite- or phyllosilicate-like minerals from pure Mg—Al or predominantly Mg—Al HT which may be similar to or iso-chemical in composition or may possess a similar chemistry with substitution of some ions as determined by the nature of Mg and/or Al added or the nature and chemical composition of the natural or wastewater which may influence the final geochemical composition, crystallinity or mineralogy.
This increased stability of LDH or HT or chlorite-like minerals or other LDH or HT derivatives may also be achieved possibly in combination with chemical methods described above by partial or complete evaporation, calcination or vitrification leading to part or complete dehydration and partial/total recrystallisation. The use of co-amendments with, or encapsulation of, the LDH or HT may also be an option to further increase physical or chemical stability.
The International Atomic Energy Agency (which is the international centre of cooperation in the nuclear field working with member states and multiple partners worldwide to promote safe, secure and peaceful nuclear technologies) published a report in 2004 summarising the state of the art in the field of treatment of effluents from uranium mines and mills. Importantly, the novelty of the invention as described herein using the addition of chemical compounds to modify solution chemistry to form LDH or HT is exemplified by the absence of any similar description or process for the treatment of effluents from uranium mines (IAEA, 2004).
The applicant does not concede that the prior art discussed herein forms part of the common general knowledge.
Throughout this specification, the word “comprising” and its grammatical equivalents is to be taken to have an inclusive meaning unless the context of use indicates otherwise.